A power converter converts an input voltage waveform into a specified output voltage waveform. In many power processing applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include an inverter, an isolation transformer and a rectifier on a secondary side of the isolation transformer. The inverter usually uses a switching device, such as a field effect transistor, that converts the DC input voltage to an AC voltage. Then, the isolation transformer transforms the AC voltage into another value, and the rectifier generates the desired output DC voltage. The rectifier may include one or more rectifying diodes that conduct the load current only when forward-biased in response to the input waveform to the rectifier.
Unfortunately, the rectifying diode suffers from a reverse recovery condition when transitioning from conduction to a non-conduction state. During this reverse recovery condition, the diode current reverses direction, causing excess energy to be stored in the leakage inductance of the isolation transformer and wasted power in the diodes. The energy stored in the leakage inductance is dissipated in a resonant manner with the junction capacitance of the rectifying diode, causing oscillation (or ringing) and overshoot in the voltage waveform of the rectifying diode. As a result, the converter suffers an efficiency loss that impairs its overall performance.
A traditional manner of reducing these rectifying diode losses is to use a snubber circuit that is coupled to the rectifying diodes. For example, the snubber circuit may be a resistor-capacitor-diode (RCD) snubber circuit. The RCD snubber circuit dampens oscillations in the rectifier voltage. During each switching transient, the reverse recovery energy due to the recovery process of the diodes is first stored in the snubber capacitor and then nearly all of the energy is dissipated in the snubber circuit resistor. As the output power increases, the power dissipated in the snubber resistor becomes significant thereby usually limiting the RCD snubber to lower power applications.
To reduce the power loss in the snubber resistor, a lossless snubber circuit may be used, which operates in a manner similar to the RCD snubber circuit. However, the energy taken into this snubber circuit is recovered to an auxiliary inductor through oscillation with a lossless snubber capacitor. This snubber circuit loses its effectiveness where small duty cycle ratios are involved, since there is insufficient time to discharge the energy stored in the snubber circuit.
An alternative approach is to employ a saturable reactor snubber circuit in series with the rectifying diode. The saturable reactor has an amorphous core that makes a transition between low impedance (i.e., saturation) and high impedance with relatively low core losses. When the rectifying diode is conducting, the saturated reactor provides low impedance thereby allowing the current to flow freely. However, when the rectifying diode transitions from conducting to a non-conducting state causing reverse recovery to occur, the reactor provides high impedance thereby reducing the reverse current flow. Unfortunately, the saturable reactor may have to be cooled by forced air to avoid overheating.
Yet another approach to manage the losses associated with the reverse recovery condition is to employ a clamp circuit coupled to the rectifying diodes. This clamp circuit limits the peak voltage and reduces the stress across components within the converter. An advantage associated with such a circuit is that a clamp circuit does not dissipate energy in the converter. Unfortunately, the clamp circuit is generally limited to applications wherein the output voltage of the converter is fixed. Modifications can be made to the clamp circuit, however, to make it independent of the output voltage. For example, a coupling transformer may be connected across the main transformer effectively recovering excess transient energy to the primary side of the transformer. Unfortunately, the coupling transformer of the clamp circuit is generally comparable in size to the main transformer.
Accordingly, what is needed in the art is an enhanced way to suppress a reverse recovery condition associated with a rectifier.